The present invention relates to a method for the preparation of a crystalline thermoplastic resin sheet or, more particularly, to a method for the stable and high-speed preparation of a crystalline resin sheet having excellent transparency, high gloss and uniformity which can be used as such for the fabrication of a paper folder and the like cardboard products or for the fabrication of various boxes and containers utilizing the excellent thermoformability thereof.
Conventional thermoplastic resin sheets of high transparency used hitherto are mostly made of a non-crystalline or amorphous resin such as a polyvinyl chloride resin, polystyrene-based resin and the like. Sheets of these amorphous thermoplastic resins have excellent thermoformability so that they are widely used in the fabrication of various boxes and containers useful in many fields. Among these amorphous thermoplastic resins, however, polyvinyl chloride resins are disadvantagenous in respects of hygiene, heat resistance, moisture proofness and other properties. Moreover, wastes of the resin cannot be disposed without the very serious problem of environmental pollution, for example, due to the emission of chlorine-containing gases in the course of incineration for disposal. Polystyrene-based resins are also not quite satisfactory in respects of the heat resistance, impact strength, moisture-proofness and other properties. Notwithstanding these disadvantages and problems, sheets of these amorphous thermoplastic resins are widely used as a packaging material in many fields solely by virtue of their excellent transparency.
Accordingly, it is a recent trend to convert the base resin of the sheets from the above mentioned amorphous thermoplastic resins to a crystalline resin having excellent mechanical strength and heat resistance. Crystalline thermoplastic resins are, of course, inferior in the transparency due to the crystallinity. Although the transparency of a crystalline thermoplastic resin sheet can be increased by quenching in cold water, appearance of haze dots is unavoidable in the sheet which has been quenched so that such a quenched sheet is no longer acceptable with its poor appearance to be utilized as such and difficulties are also encountered when such a sheet is to be used as a packaging material in the form of a box or other container.
Nevertheless, polypropylene-based resins, for example, are increasingly used in recent years in place of the polyvinyl chloride resins and polystyrene-based resins by virtue of their excellent strength, rigidity, heat resistance, moisture proofness and other properties. They are, however, far from satisfactory when transparency is an essential characteristic of the resin sheet and the applicability of these resins is greatly limited also owing to their relatively low rigidity and thermoformability in comparison with the above mentioned amorphous thermoplastic resins. In particular, the low transparency so far thereof is the determinant factor for the absence of competitiveness with the amorphous resins.
Various attempts and proposals have been made hitherto to improve the transparency of a polypropylene sheet. In connection with the quenching method of an extruded sheet of a molten resin to control the crystalline structure thereof, for example, following methods have been proposed.
The first is the chill roll method which is, however, not suitable when quenching is desired down to a temperature below the dew point because dew drops are deposited on the surface of the chill roll which is at a temperature below the dew point. In addition to the above mentioned limitation in respect of quenching, no resin sheet of excellent surface condition can be obtained by this method because air is unavoidably caught between the roll surface and the running molten resin sheet, especially, in high-speed molding.
The second of the proposals is the water quenching method. Although a high quenching effect can be obtained in a conventional water quenching method, the difficulty in obtaining uniformity in the quenching effect frequently results in the appearance of boiling dots and haze dots in the quenched resin sheet. Therefore, sufficiently uniform resin sheets can be obtained only at an extremely low molding velocity.
Thirdly, the method of slit water quenching has been proposed which, although the method is advantageous in respect of the efficiency of quenching and uniformity of quenching, is not always quite satisfactory because the method is not suitable for highspeed molding and not free from the problems of haze dots and stripes, unevenness in the thickness of the sheet, macroscopic undulation on the surface of the sheet and curling of the sheet.
Furthermore, following methods have been proposed for the improvement of the transparency of polypropylene resin sheets.
Thus, the fourth method is the use of a resin admixed with a nucleating agent but this method is not free from the problems of bleeding of the nucleating agent on the surface of the sheet as well as the unpleasant or offensive odor and toxicity thereof in addition to the limited improvement of the transparency.
The fifth method is the use of a resin blended with a petroleum resin. This method is, however, disadvantageous due to the decrease in the heat resistance and moisture proofness inherent to the polypropylene resins in addition to the limited improvement of the transparency of the sheet.
For these reasons, the trend of the technology is in the direction of improving the transparency of the containers shaped of the resin sheet by thermoforming and not in the direction of improving the transparency of the polypropylene resin sheet per se.
The sixth method, i.e., Japanese Patent Publication 57-17689 discloses a method for the fabrication of a transparent container of a polypropylene resin sheet by pressure forming at a temperature lower than the melting point thereof after heating of the resin sheet at a temperature of the melting point or higher followed by quenching. In this method, however, the resin sheet shaped in advance is reheated so that disadvantages are unavoidable in the degradation of the resin by the second heating and unevenness in heating if not to mention the costs for the increased energy consumption. Moreover, another problem is in the difficulty of obtaining uniform quenching which results in an insufficient degree of improvement in the transparency of the sheet and formation crimps and slackenings in the resin sheet in the course of the thermoforming.
A seventh method has been proposed in which a thermoplastic resin sheet is imparted on both surfaces with a surface roughness of 0.7 .mu.m RMS or smaller and the resin sheet is unidirectionally stretched in a stretch ratio of 3 times or less followed by thermoforming (see, for example, Japanese Patent Kokai No. 53-128673). This method, however, has no contribution at all to the improvement of the internal haze in addition to the difficulty in reducing the surface roughness on both of the surfaces of the sheet. Therefore, the resin sheet treated by this method cannot have a sufficiently improved transparency. Moreover, this method is effective in the improvement of the transparency only when the stretch ratio is as high as 1.5 to 2.5 times while such a high stretch ratio is detrimental greatly to the thermoformability of the resin sheet. In addition, the transparency can be increased only by a deep drawing to some extent so that tearing of the sheet in the MD direction and non-uniformity in the transparency are unavoidable as a result of the unevenness in drawing. The resin sheet obtained by this method has a relatively low heat resistance due to the orientation therein so that the sheet is not suitable for fabrication by vacuum forming which necessitates a relatively high thermoforming temperature.
As is understood from the description given above in detail, in the prior art, there are known absolutely no sheet products of a crystalline thermoplastic resin or, in particular, polypropylene-based resin having excellent transparency and surface properties along with low orientation and excellent thermoformability.